1. Field of the Invention
The present invention relates to carbon composite materials, and more particularly to carbon composite materials that include silicon carbide and have improved oxidation resistance and carbon friction performance in humid environment.
2. Description of Related Art
It is well known that carbon-carbon composites possess excellent mechanical properties at high temperatures, as well as having a low coefficient of thermal expansion and a high thermal conductivity. See, e.g., Rubin, L., in Carbon-carbon materials and composites, Buckley et al., Eds., p.267, Noyes Publ., NJ (1993); Fitzer et al., in Petroleum derived carbons, Bacha et al., Eds., p.346, American Chemical Society (1986); Sheehan, J., in Carbon-carbon materials and composites, Buckley et al., Eds., p.223,Noyes Publ., NJ (1993); and Fitzer, E., Carbon, 25:163 (1987). Such properties make these materials attractive for such uses as airframe structures, aerospace engines and brake components. One drawback, however, is that carbon is oxidized in air at temperatures as low as 400xc2x0 C., and in some applications, even a small amount of carbon oxidation can compromise the utility of a carbon-carbon composite part. Since carbon composite materials hold such great promise as materials of construction for demanding applications, a great deal of research has been focused on the development of methods to improve their oxidation resistance. See, e.g., Sheehan, J., Carbon, 27(5):709 (1989).
Previous efforts to improve the oxidation resistance of carbon-carbon composite materials have been directed to two major areas. One area of research has centered around the application of oxidation-resistant coatings, such as silicon carbide, to block oxygen contact with the oxidation-prone carbon. This work has been discussed by Sheehan, in Carbon-carbon materials and composites, Buckley et al., Eds., p.223, Noyes Publ., NJ (1993); Strife et al., Ceramics Bulletin, 67(2):369 (1988); Wu et al., Carbon, 29(8):1257 (1991); and in U.S. Pat. Nos. 4,668,579, 4,671,997, 5,298,311, 5,380,556, 5,536,574, 5,736,232, and 5,752,773 among others. The major problem with the use of coatings is that the coatings usually induce stresses in the fabricated part due to the different coefficient of thermal expansion of the coating material and the carbon composite. This often leads to crack formation. See, Sheehan, J., Carbon, 27(5):709 (1989). It has been reported that such cracks can be minimized, or sealed, if the protective coating is used in conjunction with another layerxe2x80x94for example, a glass coatingxe2x80x94which can seal the cracks as they develop. See, e.g., Sheehan, in Carbon-carbon materials and composites, Buckley et al., Eds., p.223, Noyes Publ., NJ (1993); and Liu et al., J. Mater. Sci. Lett., 12(12):886 (1993). But, such additional coatings can be expensive to apply and require more complex fabrication techniques. Moreover, if even a small crack remains unsealed, oxidation that is initiated at that point can severely damage the overall integrity of the carbon composite part.
The other general method for protecting carbon composites is the use of matrix inhibitors, such as boron or boron carbide. These materials reduce carbon oxidation by spreading a sealantxe2x80x94borate glassxe2x80x94within the composite. See, e.g., Sheehan, id. at p.223, as above; and Liu, id., as above. It has been shown, however, that because of their relatively low melting point, such inhibitors introduce temperature limitations for composite applications and are effective only after an appreciable fraction of carbon has been gasified. See, e.g., Fitzer, E., Carbon, 25:163 (1987); McKee, D. W., Carbon 26(5):659 (1988); Sheehan, J., Carbon, 27(5):709 (1989); Strife et al., Ceramics Bulletin, 67(2):369 (1988); and Wu et al., Carbon, 29(8):1257 (1991). This is unacceptable in certain applications, since as little as a few percent of weight loss can drastically reduce the mechanical properties of the composite.
Other methods for the protection of carbon composites include the addition of oxidation resistant materials such as polycrystalline silicon carbide particles (Chin, A., et al., Proc. Mater. Res. Soc., Boston, Mass., p.106, Nov. 28 (1994)), and chemical vapor infiltration (CVI) is often used for this purpose. However, the structure that is produced is prone to crack formation due to the different coefficients of thermal expansion of the carbon and the relatively large polycrystalline silicon carbide particles.
Another study reported the formation of crystalline silicon carbide from silicon carbide precursors in association with carbon precursors. Kawamura et al., in Carbon, 30(3):429 (1992), reported that silicon carbide/carbon composite sheets that were produced from a silicon-containing polymer and heat-treated coal tar pitch gave promising results in terms of oxidation resistance and mechanical strength. However, the study was limited to the formation of silicon carbide/carbon composites from mixtures or emulsions of finely ground solid precursors which gave composites having weight ratios of silicon carbide-to-carbon matrix material of over 1.5/1.
Several studies have reported the properties of carbon/silicon carbide/carbon composites that were prepared by CVI techniques. However, it was found that the silicon carbide that formed in the matrix was polycrystalline, and, as mentioned above, such polycrystalline regions can induce mechanical stresses during thermal cycling. The composites that were prepared by CVI with co-deposition of silicon carbide with carbon exhibited a lower oxidation rate (Kim, et al., Carbon, 31 (7):1031 (1993)), and similar or improved mechanical properties as compared with pure carbon-carbon composites (Park, et al., Carbon, 30(6):939 (1992)).
These studies underscore the positive role of silicon carbide within the carbon matrix on improving the oxidation resistance of carbon/carbon composite materials. However, presently available carbon composites containing silicon carbide at levels that improve oxidation resistance often sacrifice strength and resistance to thermal stress. Accordingly, there is a need for carbon-carbon composite materials that demonstrate improved oxidation resistance at higher temperaturesxe2x80x94and in particular an oxidation resisting effect that is not limited to the surface, but is distributed throughout the bulk of the composite; and also for such a material that sacrifices a minimum amount of physical strength and resistance to thermal stress at such higher temperatures; and also for such a material that could be easily fabricated without the need for multiple-step fabrication processes, or the application of costly protective coatings and the like.
When carbon composite materials are used in braking systems, or other applications wherein friction is an important property in determining performance, another problem has been the effect that oxygen and moisture can have upon the coefficient of friction of the composite material. It has been found that the coefficient of friction of carbon-carbon materials can be significantly reduced by the presence of condensable vapors, and that when such vapors are present, some form of lubricating film can be formed on the surface of the carbon material. (See, e.g., Earp, F. K., The Industrial Chemist, p. 495 (Oct. 1961); Ramadanoff, D., and S. W. Glass, Trans. AIEE, 6xe2x80x94:825 (1944); Campbell, W. E., and R. KoZak, Trans. AIEE, 70:491 (1948); Zaidi, H. et al., Appl. Surf. Sci., 44:221 (1990); and Yen, B. K., J. Mater. Sci. Let., 14:1481 (1995)). Thus, the presence of moisture in contact with a carbon-carbon brake surface can reduce the effectiveness of the brakes very significantly, and as little as the amount of water vapor present in the air can be sufficient to cause this effect. Some braking systems even have to be heated to prevent the presence of moisture.
Accordingly, it would be useful to provide a carbon composite material that not only would have advantageous oxidation resistance without sacrificing strength, as described above, but would also be less susceptible to the lubricating effects of moisture.
Briefly, therefore, the present invention is directed to a novel oxidation resistant carbon composite material comprising nanocrystalline silicon carbide regions distributed throughout a carbon matrix.
The present invention is also directed to a method for preparing an oxidation resistant carbon composite material comprising forming a solids-free solution of a silicon carbide precursor and a carbon precursor in a solvent; removing the solvent; and pyrolyzing the material remaining after removing the solvent, thereby forming nanocrystalline silicon carbide in a carbon matrix.
The present invention is also directed to a novel method for preparing an oxidation resistant carbon composite material comprising intermixing in a solvent a silicon carbide precursor and a carbon precursor and forming a solution that is free of solids; removing the solvent; and pyrolyzing the material remaining after removal of the solvent, thereby forming nanocrystalline silicon carbide in a carbon matrix.
The present invention is also directed to a novel oxidation resistant carbon composite material that has been prepared by mixing a silicon carbide precursor with a carbon precursor in a solvent to form a solution that is free of solids; removing the solvent; and pyrolyzing the material remaining after removal of the solvent, thereby to form silicon carbide in a carbon matrix.
The present invention is also directed to a part that comprises a novel oxidation resistant carbon composite material comprising nanocrystalline silicon carbide regions distributed throughout a carbon matrix.
Among the several advantages found to be achieved by the present invention, therefore, may be noted the provision of a carbon-carbon composite material that demonstrates improved oxidation resistance at higher temperaturesxe2x80x94and in particular an oxidation resisting effect that is not limited to the surface, but is distributed throughout the bulk of the composite; the provision of such a material that sacrifices a minimum amount of physical strength and resistance to thermal stress at such higher temperatures; the provision of such a material that could be easily fabricated without the need for multiple-step fabrication processes, or the application of costly protective coatings and the like; and the provision of such a material that is less susceptible to the lubricating effects of moisture.